Add TODO compile var + Fix doxygen issue

You can make your code warn on compile time for the TODO's
In order to do so, uncomment the following line. in pro file
DEFINES += TODO_LIST

sources/qetapp.cpp

@@ -1494,7 +1494,9 @@ void QETApp::useSystemPalette(bool use) {
 */
 void QETApp::quitQET()
 {
+#if TODO_LIST
 #pragma message("@TODO Segmentation fault when closing program before loading elements is finished")
+#endif
 	if (closeEveryEditor()) {
 		qApp->quit();
 	}